INITIAL PLANT MATERIAL AND THE MAIN PALEOGEOGRAPHIC CONDITIONS OF FORMATION OF THE BROWN-COAL SEAMS OF THE TATAUROVO DEPOSIT, CHITA OBLAST

A. V. Aksarin, O. A. Dulzon, V. L. Kokunov

The Tataurovo brown-coal deposit is located 65 km southwest of the city of Chita, within the territory of the Uletovsky District of Chita Oblast. The deposit is confined to the central part of the Chita-Ingoda depression. The Chita-Ingoda depression is a narrow intermontane basin of northeastern strike, filled with Meso-Cenozoic continental sedimentary deposits. The Chita-Ingoda depression is the largest depression of Central Transbaikalia; from the northwest it is bounded by the Yablonovy Range, and in the southeast by the Chersky Range [2].

The principal objective of the present work is to study the conditions under which the coal seams formed, the changes in their composition in time and space, and the floristic composition of the coal-forming plants of the Tataurovo coal basin.

In the course of the geological history of the formation of the deposit, plant remains were subjected to processes of gelification and fusenization, as a result of which the Tataurovo coals contain mainly lignin-cellulose elements in the form of gelified and fusenized components, while leiptinite elements occur only in insignificant quantities. The brown coals studied from the workable seams of the Tataurovo deposit are composed chiefly of remains of higher plants. Remains of lower plants occur in them extremely rarely.

One of the most important factors of coal accumulation was the paleogeographic setting during sedimentation of the coal-bearing strata. Of no less importance was the character of the geotectonic movements that accompanied sedimentation and coal accumulation. These factors were closely connected with the development of vegetation as the parent material of coal accumulation.

By the Lower Mesozoic stage of coal accumulation, terrestrial vegetation had acquired decisive importance in coal formation over the vast territory of the Angara continent.

A. N. Krishtofovich believed that the Mesozoic flora of Angarida developed, on the one hand, from sagas of the ancient Angaran Permo-Carboniferous flora with an admixture of Gondwanan elements, and, on the other, from the remnants of certain representatives of the Upper Permian and Lower Triassic xerophytic flora that had survived from the west [3]. Of the horsetails, the ancient Mesozoic Neocalamites, which developed on soils ...

from Gondwanan predecessors; among ancient ferns the Dipteridiaceae appear, from the humid regions of China and Korea. Of the latter, Hausmannia and Dictyophyllum were destined to survive throughout the entire Jurassic and reach the Lower Cretaceous, whereas Clathropteris disappeared at the beginning or in the middle of the Jurassic. The ubiquitous forms here, capable of living under highly varied conditions, were Cladophlebis denticulata, C. haiburnensis, Ginkgo digitata, Phoenicopsis angustifolia, Czekanowskia rigida, and Podozamites lanceolatus. A special feature in this region, not observed in other countries, is the presence and important role of the genera Czekanowskia and Phoenicopsis. V. D. Prinada proposed calling this flora Siberian, and distinguishing the territory occupied by it as the Siberian floristic region [6]. E. M. Markovich et al. attempted to reconstruct not only the distribution of paleofloristic regions and provinces, but also the distribution of vegetation types [4]. They distinguished a zone of conifer–taiga forests, corresponding in its outlines to the Siberian paleofloristic region, within which areas are indicated for the development of coniferous forests on uplands and mountains, conifer–taiga forests and fern thickets on plains, horsetail–fern thickets, and mixed forests on swampy plains. Further, a zone of mixed cycadophyte–ginkgo–conifer forests is distinguished, corresponding to the transitional zone between the Siberian and Indo-European regions. Within this zone, various types of vegetation are shown, confined to uplands and lowlands. However, V. A. Vakhrameev, while approving the attempt to restore vegetation types, expresses regret that E. M. Markovich et al. did not show the methodology by which they reconstructed the vegetation types they distinguished [1].

Fossil plants can serve, and do serve, as exceptionally valuable indicators of the setting in which these plants lived, of climate and landscape, of the age of the deposits characterized by them, of the initial materials from which various combustible minerals were formed, and even of the quality of the latter.

Regarding the question of coal accumulation in the territory under study, it is necessary to dwell briefly on the characteristics of the Siberian botanical-geographical province, which predetermined the process of coal formation.

In the territory studied, Mesozoic coal accumulation began in the Early Triassic and continued until the epoch of the Late Jurassic and Early Cretaceous. The moderately humid and warm climate of the Siberian province was optimal for the growth of the flora that caused powerful peat accumulation here.

In Jurassic time, the development of the vegetation cover in Eastern Siberia and Transbaikalia took place under conditions of repeated changes in the external environment, which caused a corresponding restructuring of floristic complexes. As a result, in individual intervals of geological time there existed here combinations of plant forms characteristic only of that particular moment of geological history. And although each individual plant species had a fairly broad vertical distribution, their combinations and proportions in floristic complexes changed in a definite way, representing an irreversible phenomenon determined by the course of changes in the conditions of plant growth. It is precisely this mobile aspect in the development of the Jurassic flora—rather uniform in systematic terms—which tends to escape the attention of researchers and which chiefly determines its biostratigraphic possibilities.

The study of paleobotanical materials and of the regularities in the change of floristic complexes through time is of especially great importance

has for the study of continental coal-bearing deposits of Transbaikalia, where the latter have been preserved as separate spots isolated from one another. Here, in sections of coal-bearing strata, guide faunal remains are usually absent, and in correlating deposits and determining the age and conditions of coal accumulation, remains of leaf flora and spore-pollen complexes sometimes serve as the only criterion. Unfortunately, the Mesozoic floras of Transbaikalia are still little studied; however, analysis of the available materials makes it possible to say that the flora here remained of one type over a considerable interval of time, that is, throughout the entire Middle and Upper Jurassic. It contains a derivative of the flora of the Irkutsk coal basin.

In Transbaikalia, as V. D. Prinada believes, the core of the Siberian flora occurs in Middle Jurassic deposits, between the Aalenian and Bathonian, which seems quite probable to us [6]. The same opinion is held by G. G. Martinson, who established Middle Jurassic deposits in the section of the Gusinoozersk depression on the basis of a study of the freshwater mollusks Ferganoconcha [5], but V. A. Vakhrameev is inclined toward a significant rejuvenation of the Transbaikalian Mesozoic plant complex to the Early Cretaceous [1].

In Central Transbaikalia, which is of interest to us, the Mesozoic plants of the Chernovskoe brown-coal deposit have been studied most fully. Here 10 species of plant remains have been established, which have few forms in common with the rich flora of the Irkutsk coal basin; but this cannot be surprising if the Chernovskoe plant remains are assigned to the Middle or, all the more, Upper Jurassic, since recently the Ust-Balei complex of the Irkutsk basin has been accepted as Liassic. V. D. Prinada, taking the Ust-Balei flora as Middle Jurassic, believed that the Chernovskoe plants from the coal-bearing deposits would be younger. We think that it will not be a mistake to accept the Chernovskoe plant complex as Middle Jurassic or, in the extreme case, Upper Jurassic.

The flora collected and identified by us from the coal-bearing deposits of the Tataurovo brown-coal deposit, adjacent to the Chernovskoe brown-coal deposit of Central Transbaikalia, comprises 20 species: Phoenicopsis stobieckii Racib., Ph. angustifolia Hr., Ph. speciosa Hr., Sphenobaiera magnifolia Aksarin, Pseudotorellia (Feildenia) ensiformis (Hr.) Dolud, Ps. pulchella (Hr.) Vasil., Angariella angustifolia (Hr.) Prin, Radicites sp., Coniopteris angarensis Pryn., Carpolites sp., Podozamites lanceolatus Eichw., Pityophyllum nordenskioldii (Hr.) Nath., Pityophyllum sp.

All the plants listed are representatives of the Siberian floristic region. Many of them are common in the Ust-Balei complex. Among them there are no typical representatives of Lower Cretaceous forms, nor are there representatives of the Lower Mesozoic phytosa. We believe that these plants characterize the upper parts of the Middle Jurassic or the Upper Jurassic.

The most widespread plants of the indicated plant complex are ginkgoaleans, especially Phoenicopsis, and conifers, which composed a conifer-ginkgo taiga, in the undergrowth of which ferns occurred. Ginkgoaleans and conifers, with the participation of ferns, were the principal coal-forming plants during the formation of the coal deposits of the Tataurovo field. Unfortunately, it proved impossible to use the palynological method because of the extreme poverty of the coal-bearing deposits in spores and pollen.

Thus, in resolving the question of the composition of the vegetation cover, we mainly used only the organographic method of studying the flora. Which plants were the principal suppliers of raw ...

initial plant material during the formation of peat bogs in the Chitino-Ingodinskaya depression and, in particular, at the Tataurovo deposit?

The coal-forming plants may be judged from leaf impressions or from remains of stems and trunks collected in the floor and roof of coal seams, directly at the surface of the boundary between the coal substance and the rock substance, and in rock partings within the coal seams.

Our observations in drill cores from the Tataurovo deposit showed that the roofs and floors of seams II and III are usually enriched with remains of Phoenicopsis to the greatest degree, i.e., with ginkgoalean plant forms. In the rock partings within seam I, Scleropteris is often present. This compels us to regard ginkgoalean plants as the principal supplier of plant material, at least at the beginning and at the end of the stages of formation of the coal seams of the Tataurovo deposit.

In the course of a detailed petrographic description of the coals of the principal seams of the Tataurovo deposit, we noted that these seams are composed mainly of stem and trunk parts of plants occurring in the stage of vitrinite, semivitrinite, and fusinite.

The study of transverse, radial, and tangential sections of wood occurring in the fusinite stage showed that most of them, by the character of the structure of the tracheids in transverse sections and by the presence of bordered pores with tori, usually arranged in a single row and not touching one another in tangential sections, must be assigned to the trunk parts of ancient coniferous plants. In transverse sections, this type of anatomical wood structure is characterized by clearly expressed annual rings, indicating the seasonality of the climatic setting.

Summarizing the data from the two methods of studying the composition of coal-forming plants, it may be asserted with a high degree of probability that they were chiefly woody plants. Consequently, during the formation of coal seams I, II, and III, the Tataurovo deposit represented a zone of development of conifer–ginkgo forests with a subordinate importance of other plants, but this zone was always waterlogged.

What types of bogs, then, were the site of accumulation of the Tataurovo coals?

A generalization of the data of peat science and bog science on modern bogs, carried out by us and by other researchers, shows that the genetic subdivision of bogs into lowland, raised, mixed, transitional, and floodplain bogs is a reflection of the relief and the initial phytocenosis and is justified. The lowest ash content of the peat mass is found in raised bogs (from 2 to 4%), lowland bogs (10–14%), and bogs of mixed and transitional type—within the range of 3–15%. The highest ash content of the peat mass is possessed by floodplain-type bogs and mixed lowland–floodplain bogs. Their ash content exceeds the ash content of peat from lowland bogs by 3–5% or more. Thus, the ash content of the peat mass of modern bogs is an important diagnostic feature for identifying the type of bog.

The process of formation of peat deposits in Jurassic time was in itself a rather complex process. Here, undoubtedly, the specific imprint of the composition of Jurassic plants was imposed, which cannot be compared with the composition of the modern flora, as well as the mode of nutrition of the plants, their longevity, and also the climate of that remote epoch.

Undoubtedly, in Jurassic time on the ancient continent (Angaraland) there also existed mountain ranges, down whose slopes numerous rivers flowed into the ancient epicontinental seas. Undoubtedly, on the ancient continent there were also low-lying, peneplanated areas on which, under favorable conditions, ancient lowland, raised, and floodplain bogs developed along rivers, as well as bogs of mixed and transitional types. At the present time we observe them already in a new form—as coal seams.

According to a compilation of a very considerable amount of data on peat formation, carried out by the Main Administration of the Peat Fund under the Council of Ministers of the RSFSR, each type of bog is subdivided into three subtypes: forest, forest–bog, and quagmire.

Turning to the determination of the types and subtypes of bogs in which the accumulation of the initial material of the Tataurovo coal seams took place, it should be noted that the original phytocoenosis, as well as the high ash content of the Tataurovo coals, together with the character of the relief that existed at that time, allow one to conclude that, in general, the Tataurovo coals of seams I, II, and III formed in a narrow intermontane depression; moreover, the area of coal accumulation represented an entire system of elongated floodplain-type bogs connected with one another by bogs of the lowland type. Judging by the composition of the coal-forming plants, as well as by the character of decomposition of the plant material, reflected in the identification of the corresponding petrogenetic types of coals, we are dealing with bogs of the forest and forest–bog subtypes.

LITERATURE

1. V. A. Vakhrameev. Jurassic and Early Cretaceous Floras of Eurasia and the Paleofloristic Provinces of That Time. Moscow, “Nauka,” 1964.

2. Geology of the USSR. Vol. XXXVI, Moscow, 1961.

3. A. N. Krishtofovich. Plant remains from lacustrine Jurassic deposits of Transbaikalia. Transactions of the Mineralogical Society, issue 1, 1916.

4. E. M. Markovich, Z. P. Prosviryakova, I. Z. Fadeeva. Paleogeobotanical zonality and climate of the Lower Mesozoic. In: Atlas of Maps of Coal Accumulation on the Territory of the USSR. Explanatory Note. Publishing House of the Academy of Sciences of the USSR, 1962.

5. G. G. Martinson. Mesozoic and Cenozoic mollusks of continental deposits of the Siberian Platform, Transbaikalia, and Mongolia. Proceedings of the Baikal Limnological Station, issue 19, 1961.

6. V. D. Prinada. On the Mesozoic flora of Siberia. Materials on the Geology and Mineral Resources of Eastern Siberia, issue 19, 1944.

7. V. D. Prinada. Mesozoic Flora of Eastern Siberia and Transbaikalia. Moscow, Gosgeoltekhizdat, 1962.